Wood Preservatives

without penetrating to the substrate. The pattern is usually similar to that of a crow’s foot. Cracking is larger with longer breaks extending through to the substrate (fig. 8-10). Both result from stresses exceeding the strength of the coating. But, whereas checking arises from stress within the paint film, cracking is caused by stresses between the film and the substrate. Cracking generally takes place to a greater extent on wood, due to its grain, than on other substrates. The stress in the coating is greatest across the grain, causing cracks to form parallel to the grain of the wood. Checking and cracking are aggravated by excessively thick coatings that have reduced elasticity. Temperature variations, humidity, and rainfall are also concerns for checking or cracking. WOOD PRESERVATIVES LEARNING OBJECTIVE: Upon completing this section, you should be able to describe how to treat wood for protection against dry rot, termites, and decay. There are three destructive forces against which most wood protective measures are directed: biological deterioration (wood is attacked by a number of organisms), fire, and physical damage. In this section, we’ll deal with protecting wood products against biological deterioration. Damage to wood buildings and other structures by termites, wood bores, and fungi is a needless waste. The ability of wood to resist such damage can be greatly Figure 8-10.-Severe cracking. increased by proper treatment and continued maintenance. Wood defects are also caused by improper care after preservation treatment. All surfaces of treated wood that are cut or drilled to expose the untreated interior must be treated with a wood preservative. APPLICATION METHODS There are two basic methods for treating wood: pressure and nonpressure. Pressure treatment is superior to nonpressure, but costly and time consuming. Building specifications dictate which method to use. Pressure The capacity of any wood to resist dry rot, termites, and decay can be greatly increased by impregnating the wood with a general-purpose wood preservative or fungicide. It’s important to remember that good pressure treatment adds to the service life of wood in contact with damp ground. It does not, however, guarantee the wood will remain serviceable throughout the life of the building it supports. Woods of different timber species do not treat with equal ease. Different woods have different capacities for absorbing preservatives or other liquids. In any given wood, sapwood is more absorbent than heartwood. Hardwoods are, in general, less absorbent than softwoods. Naturally, the extent to which a preservative protects increases directly with the depth it penetrates below the surface of the wood. As we just mentioned, the best penetration is obtained by a pressure method. Table 8-4 shows the ease of preservative penetration into various woods. In the table, use E for easy, M for moderate, and D for difficult. Nonpressure Nonpressure methods of applying preservatives to a surface include dipping, brushing, and spraying. Figure 8-11 shows how you can improvise long tanks for the dipping method. Absorption is rapid at first, then much slower. A rule of thumb holds that in 3 minutes wood absorbs half the total amount of preservative it will absorb in 2 hours. However, the extent of the penetration depends upon the type of wood, its moisture content, and the length of time it remains immersed. Surface application by brush or spray is the least satisfactory method of treating wood from the 8-14